In optical communications networks, transceivers are used to transmit and receive optical signals over optical fibers. On the transmit side of the transceiver, a laser of the transceiver generates amplitude modulated optical signals that represent data, which are then transmitted over an optical fiber coupled to the transmit side of the transceiver module. Various types of semiconductor lasers are used for this purpose, including, for example, Vertical Cavity Surface Emitting Lasers (VCSELs) and edge emitting lasers, which may be her divided into subtypes that include Fabry Perot (FP) and Distributed Feedback (DFB) lasers.
Various types of optics systems are used in optical transceivers for directing the light produced by the laser in one or more directions. A typical type of optics system of a known optical transceiver module includes one or more elements that direct light beams produced by the laser into the end of an optical fiber and one or more elements that direct a portion of the light produced by the laser onto one or more optical monitoring detectors, which are typically one or more monitoring photodiodes. One type of monitor photodiode is used to monitor the optical output power of the laser and produces an electrical feedback signal that is fed back to the transceiver controller. The transceiver controller processes the electrical feedback signal and adjusts the bias current of the laser, if necessary, to maintain the laser output power level at a desired average output power level.
FIG. 1 illustrates a block diagram of a portion of a transmit side of a known optical transceiver module 2 having a known optics system. The transceiver module 2 uses an edge emitting laser diode 3 as the light source for generating an optical data signal. The laser diode 3 is mounted on a substrate 4, which is normally referred to as the submount assembly of the transceiver module 2. The laser diode 3 emits an output light beam 14 from a front facet 6 of the laser diode 3 and emits a monitoring light beam 18 from a rear facet 7 of the laser diode 3. The output beam emitted from the front facet 6 is directed through an element 8 of an optics system, such as a collimating lens, which collimates the output beam. A 45° mirror 9 of the optics system reflects the collimated beam at an angle of 90° relative to the angle of incidence so that it is directed normal to the surface of the substrate 4 onto an end of a transmit optical fiber 11.
The monitoring light beam emitted from the rear facet 7 of the laser diode 3 is directed onto a detector 13, which is normally a photodiode. The detector 13 generates an electrical signal indicative of the power of the output monitoring beam. Because there is a known relationship between the power of the output beam 14 and the power of the monitoring beam 18, the signal generated by the detector 13 may be used to determine the power level of the signal directed into the end of the fiber 11. A transceiver module controller 19 processes the electrical feedback signal output from the detector 13 and outputs a control signal to the laser diode 3 that causes the laser diode 3 to adjust its bias current to maintain the output power level of the beam 14 at a particular level.
While the monitoring and controlling techniques described with reference to FIG. 1 generally operate well for their intended purpose, there is room for improvement. For example, the known ratio of the power of the two beams 14 and 18 is less reliable with respect to maintaining the output power to the fiber 11 if the output beam 14 is manipulated in a manner different than the manner in which the monitoring beam 18 is manipulated. For example, in an Externally Modulated Laser (EML), the modulation which occurs for telecommunications or other applications does not affect the monitoring beam 18. Thus, the feedback signal out from the detector 13 and provided to the controller 19 will not show all fluctuations in the output power level of the beam reflected by the mirror 9 into the end of the fiber 11.
In addition, the processes that have been used to make the 45° mirrors for use in optical transceiver modules are generally tedious, prone to human error and expensive. One known approach to fabricating a small-scale mirror is to use anisotropic etching of silicon. Certain wet etchants, such as potassium hydroxide (KOH), will etch primarily in the direction of the crystal plane. The section entitled “Description of the Related Art” in U.S. Pat. No. 6,417,107 to Sekimura describes one known etching technique for forming a 45° mirror. A silicon ingot may be sliced at an angle to obtain a <100> silicon wafer which is 9.74° off-axis. Without the oblique cut, the wet etchant would etch at an angle of 54.74°. However, the off-axis silicon substrate etches at an angle of precisely 45° (54.74°-9.74°). The etching angle is determined by the orientation of the <111> crystallographic plane, which typically has a very slow etch rate. This property enables the <111> crystallographic plane to be used as an etch stop. Thus, in the 45° mirror, the reflective surface is along the <111> crystalline plane.
There are a number of factors that affect the planarity (i.e., smoothness) of the resulting <111> crystallographic plane. Techniques have been introduced to increase the planarity. It is a common practice to add a surfactant into the etchant in order to improve surface smoothness. For example, isopropyl alcohol may be introduced into the KOH. As another, arsenic salt has been added to passivate and smooth the etched surface. The Sekimura patent describes using an etchant of KOH or tetramethylammonium hydroxide (TMAH) with a non-ion type surface active agent, such as polyoxyethylene alkyl phenyl ether. It is also known to introduce an impurity into the silicon crystal itself in order to reduce roughness on the surface.
Annealing a rough silicon surface in a reduced pressure hydrogen atmosphere can improve the smoothness of a silicon substrate. It is possible that the planarity of a mirror surface may be improved after it is formed, if the anneal is applied. During the etching process, planarity can be improved by reducing or eliminating the occurrence of bubbling on the silicon surface. Hence, either oxygen or hydrogen gas can be bubbled into the etching bath.
Other concerns in the etching of silicon to form a 45° mirror relate to the tediousness and the repeatability of the process. Photo masks are typically used in conjunction with lithography during the etching process. Accurate alignment of the photo mask to the crystal axis normally includes a two-step etching approach. The first etching step reveals the true crystal orientation. Then, the second etching step requires precisely aligning the etching mask to the revealed crystal orientation prior to etching. The process is tedious, subjective and expensive. In addition, the necessity of using a special cut silicon wafer further increases costs. Furthermore, if the crystal is not precisely aligned during the lithography process, the etching process will result in steps being formed on the mirror surface. Because the etched wafer is very fragile and the entire process requires a lot of handling of the wafer, wafer fracturing resulting in yield loss is expected.
Accordingly, a need exists for an optical transceiver module having a 45° mirror that provides improved optical power monitoring capabilities, and an improved method for making a 45° mirror.